Telephone exchanges communicate with associated subscriber stations by way of pairs of talking wires to which the switching equipment of the exchange can be connected upon the initiation of a call by the subscriber, i.e. upon closure of a loop through the two wires with the aid of the usual hook switch. At the exchange the wires are tied to respective terminals of a source of direct current, one of which is usually grounded. Thus, an interconnection of the wires by the hook switch causes current to flow through both wires whereupon a line monitor responsive to such current flow generates an output indicative of this condition.
In certain instances, as where the subscriber station is equipped with a switchboard, that station may also have a key whose operation grounds one of the talking wires so that only the wire tied to the live terminal at the exchange will carry current. The line monitor should be capable in such cases to discriminate between two conditions, i.e. the flow of current through one or both wires.
Conventional line monitors, essentially comprising current/voltage converters working into associated logic circuits, are sensitive to spurious current pulses that happen to be accidentally induced in these signal wires whereby untimely switching operations may be started. There is also a danger of overvoltage which can damage the logic circuits and other components further downstream.